The present invention relates to an image forming apparatus for forming a color image with an electro-photography method.
In an image forming apparatus such as a color printer, a color copier, a color multi-function product, and the likes for forming a color image with an electro-photography method, a plurality of image forming units is detachably disposed along a transport belt for transporting a recording medium. Each of the image forming units includes a photosensitive drum. A toner image in each color is formed on the photosensitive drum, so that the toner image is sequentially overlapped and transferred to the recording medium, thereby forming a color image (refer to Patent Reference).    Patent Reference: Japan Patent Publication No. 2001-66843
In the conventional image forming apparatus, when the image forming unit has a dimensional variance, or is disposed at a shifted position, a toner image in each color is formed at a shifted position or color shift occurs, thereby deteriorating image quality.
In order to securely prevent the color shift, in the conventional image forming apparatus, when it is detected that the image forming unit is disposed at a shifted position, a color shift detection process is performed as follows.
After an image pattern formed of a toner image in each color is formed on the transport belt, the conventional image forming apparatus detects a positional relationship of the image pattern of the toner image in each color, so that the conventional image forming apparatus determines a color shift amount in advance upon forming a color image. The color shift amount is stored as a color shift correction amount.
Accordingly, when the conventional image forming apparatus forms a color image, the toner image in each color is transferred to a position shifted by the color shift correction amount thus stored, thereby preventing the color shift in a color image.
When the image forming unit is attached or detached during power off, the color shift correction amount tends to change. Accordingly, in the conventional image forming apparatus, the color shift detection process is performed when power is turned on, in addition to when it is detected that the image forming unit is attached or detached.
In the conventional image forming apparatus, even when the image forming unit is not attached or detached during power off, the color shift detection process is performed, thereby wasting toner. Further, it takes a long period of time to start up the conventional image forming apparatus.
In view of the problem described above, an object of the invention is to provide an image forming device, in which it is possible to solve the problems of the conventional developing device. In the image forming apparatus of the present invention, it is possible to securely correct the color shift. Further, it is possible to eliminate an unnecessary color shift detection process.
Further objects of the invention will be apparent from the following description of the invention.